Catch error when environment does not have variables declared

[mwobensmith]

This stack comes from a part of the code that could use try/catch and a better warning. We can print a nice error message on how to set environment variables so that this error goes away.

Traceback (most recent call last):
  File "/Users/mwobensmith/.local/share/virtualenvs/iris_firefox-7taXFi_R/bin/iris", line 10, in <module>
    sys.exit(main())
  File "/Users/mwobensmith/.local/share/virtualenvs/iris_firefox-7taXFi_R/lib/python3.7/site-packages/moziris/scripts/main.py", line 52, in main
    init_control_center()
  File "/Users/mwobensmith/.local/share/virtualenvs/iris_firefox-7taXFi_R/lib/python3.7/site-packages/moziris/scripts/main.py", line 188, in init_control_center
    for path, dirs, files in PathManager.sorted_walk(targets_dir):
  File "/Users/mwobensmith/.local/share/virtualenvs/iris_firefox-7taXFi_R/lib/python3.7/site-packages/moziris/util/path_manager.py", line 275, in sorted_walk
    names = os.listdir(directory)
FileNotFoundError: